Image display device

ABSTRACT

In one embodiment of the present invention, luminance information of a video signal inputted thereto is weighted by a weighting section. An image characteristic value determination section, by one frame, takes an average of the weighted luminance information, thereby determining an image characteristic value of one frame. A backlight controlling section sets luminance of the backlight at a backlight luminance determination section in accordance with the image characteristic value determined by the image characteristic value determination section and performs luminance correction.

TECHNICAL FIELD

The present invention relates to a display device, in particular, animage display device including a light source and a light modulationdevice for changing luminance of a display image by attenuating lightfrom the light source in accordance with a video signal.

BACKGROUND ART

Some liquid crystal image display devices have been configured astransmission-type image display devices having light source means. Thetransmission-type image display device requires a light source becausethe liquid crystal panel (the light modulation device) thereof is thatof a non-emissive type, i.e., the liquid crystal panel itself does notilluminate. A direct-view-type liquid crystal image display device isprovided with a light irradiation section called a backlight behind theliquid crystal panel thereof. A fluorescent tube called a cold-cathodetube is generally used as a light source. A projective-type liquidcrystal image display device, popularly termed a liquid crystalprojector, receives luminance to be projected on a screen from a lamplight source such as a Halogen lamp or a metal halide lamp.

Recently, in order to make the image quality of a liquid crystal imagedisplay device higher or clear, there have been proposed variousimprovement methods for dynamically adjusting the contrast of a videosignal and the luminance of a light source in accordance with anincessantly changing video signal.

FIG. 19 illustrates a structure of a liquid crystal image display devicedisclosed in Patent Document 1. In FIG. 19, 201 represents an LCD panel;202 represents a backlight; 203 represents a backlight controllingsection; 204 represents an average luminance level detector circuit; 205represents a display controlling section; and 206 represents an input.

A video signal to be displayed on the LCD panel 201 is inputted from theinput 206 in the form of, for example, a YPbPr signal (a luminancesignal and a color-difference signal). The display controlling section205 performs control for displaying the inputted video signal on the LCDpanel 201. Specifically, the display controlling section 205 performsoperations such as the conversion of the YPbPr signal into an RGBsignal, changing an order of the video signals according to a drivingmethod of the LCD panel 201, the most suitable gamma correction for theLCD panel 201, and the like.

The backlight 202 is a light source for the LCD panel 201 to obtainluminance. The backlight controlling section 203 controls the lightmodulation of the backlight 202. The average luminance level detectorcircuit 204 detects an average luminance level of a video signal. Thebacklight controlling section 203 performs luminance control on thebacklight 202 in such a manner that the higher the detected averageluminance level, the lower the luminance of the backlight 202.

By these operations, display luminance is controlled in accordance withthe average luminance level of the video signal. Accordingly, thiseffectively prevents such a problem that a viewer of the image displaydevice feel a display image to be too bright or, reversely, too dark.Thereby, this makes it possible to display a clear display screen image.

A screen luminance (brightness) of the image display device isdetermined by the product of a light transmission of the LCD panel 201multiplied by a luminescent luminance of the backlight 202. If aconstant luminance of the backlight 202 is irrelevant to the videosignal, a gradation of the video signal to be displayed solely dependson the light transmission of the LCD panel 201. Thus, the displayperformance of the image display device is accordingly determined by adynamic range (the display performance of luminance of each of white andblack) within which the LCD panel 201 can display an image.

However, the visual display performance of the image display deviceexceeds the dynamic range (the display performance of luminance of eachof white and black) within which the LCD panel 201 can display an imageby, as stated, performing the luminance control on the backlight 202 inaccordance with the input video signal.

As for other conventional improvement methods for dynamically performingluminance adjustment (luminance control) of the backlight, for example,there is a method disclosed in Patent Document 2 (U.S. Pat. No.3,495,362).

This conventional improvement method disclosed in the patent document 2is also for detecting the average luminance level (APL) of the inputvideo signal and, in accordance with the detected average luminancelevel, controlling light intensity of the light for irradiating adisplay element.

Specifically, in a case where the detected average luminance level isequal to or less than the first predetermined value, the light intensityof the light irradiating the display element is controlled in such amanner that the light intensity is adjusted to the predetermined lowestlevel or the vicinity thereof. On the other hand, in a case where thedetected average luminance level is above the first predetermined value,the light intensity of the light irradiating the display element iscontrolled in such a manner that the light intensity is continuouslyincreased along with a rise of the average luminance level.

This makes it possible to dynamically adjust luminance according to ascene of a video image, remedy problems: lack of a brightness feeling ata bright scene and a graying of black level of a dark scene, and improvea contrast feeling. Also, in a case of the dark scene, that is, the casewhere an APL of the input video signal is in a range less than apredetermined threshold, the light intensity of the light irradiatingthe display element is controlled in such a manner that the lightintensity is adjusted to the predetermined lowest level or the vicinitythereof. This makes it possible to further remedy the problem of agraying of black level of the dark scene, and thus, improve a contrastfeeling further.

Patent Document 2 further describes the division of the luminance levelof the input video signal into a plurality of luminance levelclassifications and determination of a histogram distribution of theluminance level classifications. According to Patent Document 2, thelight intensity of the light irradiating the display element is fixed ata predetermined level according to a predetermined distribution statusin a case where the determined histogram distribution of each dividedclassification is in the predetermined distribution status.

This makes it possible to extract more precisely characteristics of ascene of a video image, which are not simply determined solely from theresult of detection of APL, by controlling the light intensity of thelight irradiating the display element based on the histogramdistribution. In this way, the quality of the display image can beimproved by controlling the light intensity of the light irradiating thedisplay element more properly according to the characteristics of thescene of the video image.

Besides, Patent Document 3 (U.S. Pat. No. 3,215,388) and Patent Document4 (U.S. Pat. No. 3,513,312) also disclose improvement methods fordynamically controlling both the contrast adjustment of the video signaland the luminance adjustment of the backlight with correlationtherebetween, there are such methods, for example. According to theconventional improvement method disclosed in Patent Document 3, thedynamic range of the video signal is widened referring to the averageluminance level. A level of the video signal is shifted depending on anoffset. Because this causes a visual luminance level on a display screento shift, the lighting of a backlight is controlled in order that theaverage luminance level at image display may be equivalent to theaverage luminance level of the video signal, so that the shift isabsorbed by the light modulation of the backlight. These operationsremedy a visual contrast feeling.

According to the conventional improvement method disclosed in PatentDocument 4, both an amount of illumination light of the backlight and anamount of light extinction of the liquid crystal panel are controlledwith weighting of the video signal in accordance with the averageluminance level of the input video signal. That is, an image with highdegree of freedom in expressions is provided by controlling thebacklight and the liquid crystal panel in such a manner that the amountof illumination light of the backlight is increased in order to increasethe luminance of the display image, whereas the amount of lightattenuation of the liquid crystal panel is decreased. In addition, thisimprovement method provides an image whose variation range of the lightintensity exceeds the dynamic range of the liquid crystal panel bycontrolling the backlight in such a manner that the amount ofillumination light is decreased in order to decrease the luminance ofthe display image in the liquid crystal panel whose light attenuation isincreased, or that the amount of illumination light is increased inorder to increase the luminance of the display image in the liquidcrystal panel whose light attenuation is decreased.

[Patent Document 1] Japanese Unexamined Patent Publication, Tokukaihei,No. 8-201812 (published on Aug. 9, 1996))

[Patent Document 2] Japanese Patent (No. 3495362 (issued on Feb. 9,2004))

[Patent Document 3] Japanese Patent (No. 3215388 (issued on Oct. 2,2001))

[Patent Document 4] Japanese Patent (No. 3513312 (issued on Mar. 31,2004))

[Patent Document 5] Japanese Patent (No. 3583124 (issued on Oct. 27,2004))

DISCLOSURE OF INVENTION

There is a problem that the luminance control which is appropriateenough to the characteristics of the image cannot be performed inconventional arrangements such that the backlight is controlled inaccordance with the average luminance level of the video signal, asdescribed in the patent documents 1 to 4.

That is, the average luminance level of an image consisting of a darkesthalf-screen part and a brightest half-screen part is the same as that ofa gray (50%) full-screen image. Accordingly, the luminance control ofthe backlight in accordance with the average luminance level isperformed on each of the two types of images in the same way. However,the control according to each of the images should naturally differbecause the image consisting of the darkest half-screen part and thebrightest half-screen part, and the gray (50%) full-screen image arecompletely different in the characteristics of the images. Therefore,control according to each of the images naturally differs. Desired is anarrangement which makes it possible to perform the luminance control onsuch images, whose average luminance levels are the same, according toeach image based on the characteristics thereof.

On the other hand, as Patent Document 2 describes, an arrangement suchthat the backlight is controlled based on the histogram distribution ofthe video signal makes it possible to extract the characteristics of theimage which are not simply determined solely by the detected averageluminance level. However, a conventional luminance control of abacklight utilizing histogram distribution requires complex processingand the large calculation amount for pattern recognition because thepattern of determined histogram distribution needs to be recognizedthrough image processing.

In view of the problems, an object of the present invention is toprovide an image display device which can perform, according to thecharacteristics of each image, appropriate control on the images onwhich the same control has been performed in the conventional luminancecontrol utilizing the average luminance level and also appropriatecontrol according to the characteristics of the image through a simplerprocessing in comparison with the conventional luminance controlutilizing the histogram distribution.

In order to attain the object, an image display device of the presentinvention including a light source and a light modulation device whichattenuates light from the light source in accordance with a video signalso as to change the luminance of a display image, the image displaydevice, includes: weighting means for weighting luminance information ofthe video signal inputted thereto; image characteristic valuedetermining means for determining an image characteristic value of oneframe by taking by one frame an average of the luminance informationweighted by the weighting means; and light source controlling means forperforming luminance correction of the light source in accordance withthe image characteristic value determined by the image characteristicvalue determining means.

According to this arrangement, the weighting means weights the luminanceinformation of the inputted video signal; the image characteristic valuedetermining means determines the image characteristic value by taking byone frame an average of the luminance information weighted by theweighting means; and the light source control means performs theluminance correction of the light source in accordance with thedetermined image characteristic value.

This makes it possible to perform luminance control according to thecharacteristics of each image because, in such a way, on the basis ofthe image characteristic values obtained by weighting the luminanceinformation of the video signals and thereafter taking averages of theweighted luminance information, it is possible to distinguish evenimages which are indistinguishable from each other on the basis of theaverage luminance levels (APL) thereof, such as the image consisting ofthe darkest half-screen part and the brightest half-screen part, and thegray (50%) full-screen image.

That is, it is possible to distinguish differences between images whichcannot be simply determined solely from the detected average luminancelevels, not by the means for determining the histogram distribution andrecognizing the distribution pattern thereof, but by a simple means fortaking an average. This makes it possible to perform further effectiveluminance control according to the characteristics the images.

In order to solve the problems above, another image display device ofthe present invention is an image display device which displays an imageof an inputted video signal on a light modulation device having a lightsource, including: histogram determining means for dividing luminanceinformation of one frame of the inputted video signal into a pluralityof luminance information classifications and determining histogramdistribution of the luminance information classifications; weightingmeans for weighting the histogram distribution of the luminanceinformation classifications, the histogram distribution being determinedby the histogram determining means; image characteristic valuedetermining means for determining an image characteristic value bytaking an average of a result of the histogram distribution of theluminance information classifications, the histogram distribution beingweighted by the weighting means; and light source controlling means forperforming luminance correction of the light source in accordance withthe image characteristic value determined by the image characteristicvalue determining means.

According to this arrangement, the histogram detecting means divides theluminance information of the inputted video signal into the plurality ofluminance information classifications and determines the histogramdistribution of the luminance information classifications; the weightingmeans weights the histogram distribution of the luminance informationclassifications, determined by the histogram detecting means; the imagecharacteristic value determining means determines the imagecharacteristic value by taking an average of the result of the weightedhistogram distribution of the luminance information classifications; andthe light source control means performs the luminance correction of thelight source in accordance with the determined image characteristicvalue.

This makes it possible to perform luminance control according to thecharacteristics of each image because, in such a way, on the basis ofthe image characteristic values obtained by weighting the results of thehistogram determination and thereafter taking averages thereof, it ispossible to distinguish even images which are indistinguishable fromeach other on the basis of the average luminance levels (APL) thereof,such as the image consisting of the darkest half-screen part and thebrightest half-screen part, and the gray (50%) full-screen image.

In addition, it is possible to distinguish differences between suchimages by a simple means for taking an average of the result of thehistogram determination in comparison with the conventional arrangementsuch that the luminance control is performed through the recognition ofa distribution pattern of the result of the histogram determination.This makes it possible to perform an effective luminance controlaccording to the characteristics the images.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of one embodiment of the present invention,illustrating a structure of a substantial part of an image displaydevice.

FIG. 2 is a graph showing relation between a video signal level and anoutput of one function which a weighting section of the image displaydevice uses for weighting a video signal.

FIG. 3 is a graph showing relation between a video signal level and anoutput of another function which the weighting section of the imagedisplay device uses for weighting a video signal.

FIG. 4 is a graph showing relation between a video signal level and anoutput of still another function which the weighting section of theimage display device uses for weighting a video signal.

FIG. 5 is a graph showing relation between an image characteristic valueand a backlight luminance in one arithmetic expression (or a table)which a backlight luminance determination section of the image displaydevice uses for the determination of the backlight luminance.

FIG. 6 is a graph showing relation between an image characteristic valueand a backlight luminance in another arithmetic expression (or a table)which the backlight luminance determination section of the image displaydevice uses for the determination of the backlight luminance.

FIG. 7 is a graph showing relation between an image characteristic valueand a backlight luminance in still another arithmetic expression (or atable) which the backlight luminance determination section of the imagedisplay device uses for the determination of the backlight luminance.

FIG. 8 is a graph showing relation between an image characteristic valueand a backlight luminance in yet another arithmetic expression (or atable) which the backlight luminance determination section of the imagedisplay device uses for the determination of the backlight luminance.

FIG. 9 is a graph showing a functional capability of a temporal filterof the image display device.

FIG. 10 is one drawing of an example in which the weighting of a videosignal is performed only on the luminance information corresponding to aspecific area in a full screen.

FIG. 11( a) is another drawing of an example in which the weighting of avideo signal is performed only on the luminance informationcorresponding to a specific area in the full screen.

FIG. 11( b) is still another drawing of an example in which theweighting of a video signal is performed only on the luminanceinformation corresponding to a specific area in the full screen.

FIG. 11( c) is yet another drawing of an example in which the weightingof a video signal is performed only on the luminance informationcorresponding to a specific area in the full screen.

FIG. 12 is a block diagram of another embodiment of the presentinvention, illustrating a structure of a substantial part of an imagedisplay device.

FIG. 13 is a graph showing relation between an image characteristicvalue and a backlight luminance in an arithmetic expression (or a table)which a backlight luminance determination section of the image displaydevice uses for the determination of the backlight luminance.

FIG. 14 is a block diagram of still another embodiment of the presentinvention, illustrating a structure of a substantial part of an imagedisplay device.

FIG. 15 is a histogram showing a histogram distribution of an exampleimage determined by a histogram determination section of the imagedisplay device.

FIG. 16 is a table showing image characteristic values: the averages ofthe results which are the histogram distribution of FIG. 15 weighted bya weighting section, together with conventional average luminance levelsas a comparative example.

FIG. 17( a) is a table showing image characteristic values: the averagesof the results which are the histogram distribution, weighted by theweighting section, of an image consisting of a darkest half-screen partand a brightest half-screen part, together with the conventional averageluminance levels as a comparative example.

FIG. 17( b) is a table showing image characteristic values: the averagesof the results which are the histogram distribution, weighted by theweighting section, of a gray (50%) full-screen image, together with theconventional average luminance levels as a comparative example.

FIG. 18 is a block diagram of yet another embodiment of the presentinvention, illustrating a structure of a substantial part of an imagedisplay device.

FIG. 19 is a block diagram illustrating a structure of a substantialpart of a conventional image display device.

EXPLANATION OF NUMERALS

-   11 LCD panel (Light modulation device)-   12 Backlight (Light source)-   13 Backlight controlling section (Light source control means)-   19 Weighting section (Weighting means)-   20 Image characteristic value determination section (Image    Characteristic Value Determining Means)-   21 Backlight luminance determination section-   22 Signal level correction section (Video signal level correction    means)-   23 Illuminance sensor

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below with referenceto the figures.

Embodiment 1

An image display device of Embodiment 1 of the present invention isdescribed with reference to FIGS. 1 through 10, 11(a), 11(b), and 11(c).

FIG. 1 is a block diagram illustrating a structure of the image displaydevice of Embodiment 1 of the present invention. As illustrated in FIG.1, the image display device includes an LCD panel (a light modulationdevice) 11, a backlight (a light source) 12, a backlight controllingsection (a light source controlling section) 13, a display controllingsection 14, a weighting section (a weighting means) 19, an imagecharacteristic value determination section (an image characteristicvalue determining means) 20, and an input 16.

A video signal to be displayed on the LCD panel 11 is inputted from theinput 16 in the form of, for example, a YPbPr signal. The displaycontrolling section 14 performs control in order to display the videosignal on the LCD panel 11 and outputs the video signal to the LCD panel11 as a panel driving signal. Specifically, operations such as theconversion of a YPbPr signal into an RGB signal, changing an order ofvideo signals according to a driving method of the LCD panel 11, and themost suitable gamma correction for the LCD panel 11. A display mode ofthe LCD panel 11 is, for example, a vertical alignment mode.

The backlight 12 is a light source for the LCD panel 11 of atransmission-type to obtain screen luminance. The backlight 12 may be acold-cathode tube, a hot-cathode tube, an LED, an electron-emission typeflat surface source, or the like.

The backlight controlling section 13 outputs a backlight driving signaland thereby controls luminance of the backlight 12. The backlightcontrolling section 13 is provided with a backlight luminancedetermination section 21, a temporal filter 17, and a PWM convertingsection 18.

In accordance with an image characteristic value inputted from the imagecharacteristic value determination section 20, the backlight luminancedetermination section 21, as described below, determines the luminanceof the backlight 12 and outputs the luminance as a backlight luminancesignal. The temporal filter 17 makes change of luminance of thebacklight 12 gradual. The PWM converting section 18 outputs a backlightdriving signal in accordance with the backlight luminance signalinputted from the backlight luminance determination section 21. By theprovision of the temporal filter 17 between the backlight luminancedetermination section 21 and the PWM converting section, the occurrenceof flickers can be avoided because the luminance of the backlight 12changes slowly (through several frames) and gradually even if thebacklight luminance signal drastically changes between adjacent framesof one video signal.

The weighting section 19 is for weighting the luminance information ofan inputted video signal (hereafter an input video signal). Weighting isperformed in such a manner that luminance information of higherluminance side is weighted relatively more than luminance information oflower luminance side.

The image characteristic value determination section 20 is for, by aframe, accumulating the luminance information weighted by the weightingsection 19 and taking an average thereof (an image characteristicvalue). The backlight controlling section 13 performs luminance control(luminance correction) of the backlight 12 in accordance with the imagecharacteristic value determined by the image characteristic valuedetermination section 20.

The image display device of the present invention is characterized inthat the luminance information of the input video signal is weighed andthe weighted luminance information is averaged to find an average valuethereof as the image characteristic value, and the luminance of thebacklight 12 is determined in accordance with the image characteristicvalue. The luminance control on the backlight 12 is described below indetail.

The weighting section 19 performs weighting calculation on the luminanceinformation of all the pixels in one frame of an input video signal(corresponding to pixel values). A weighted value is a function of theluminance information of an input video signal. Because the luminanceinformation of a video signal is expressed as a signal level, thefunction of luminance information is a function of the signal level of avideo signal.

As described, in accordance with the signal level indicating theluminance information of each pixel which constitutes the input videosignal, the weighting is performed, in a case of a high signal level(luminance information is high: bright), with a heavier weight than in acase of a low signal level (luminance information is low: dark). Inother words, in accordance with the signal level of the input videosignal, a lighter weight is used in the case of the low signal level fordark than in the case of the high signal level for bright.

FIGS. 2 through 4 are graphs of functions used by the weighting section19 for weighting input video signals. The horizontal axes of FIGS. 2through 4 represent signal levels of an input video signal. A signallevel of an input video signal is expressed in percentage. A signallevel is 0% in the case of a black pixel whose pixel value of an inputvideo signal is the lowest. A signal level is 100% in the case of awhite pixel whose pixel value of an input video signal is the highest.On the other hand, the vertical axes of FIGS. 2 through 4 representoutputs: weighted pixel values of input video signals. An output isexpressed by a value between 0 and 100. An output is 0 in the case of asignal level of an input video signal is 0%. An output is 100 in thecase of a signal level of an input video signal is 100%.

In FIGS. 2 through 4, lines b represented by dashed lines represent aconventional function for taking an average of a signal level of aninput video signal as it is. On the other hand, in FIGS. 2 through 4,curved lines a1 through a3 represented by continuous lines are graphs offunctions (three types) used by the weighting section 19.

The curved line a1 shown in FIG. 2 represents a function plotting a linewhich is curved downward in relation to the line b over the whole rangeof signal levels between 0% and 100%. Assume an inflexion point is setto signal level 50%. Within a range equal to or less than signal level50%, an output is gradually increased up to 25 (½ of signal level 50%)while suppressing the increase of the output in relation to the increaseof a signal level. In a range more than signal level 50%, the output isincreased in order that the output may be 100 at signal level 100%raising the ratio of the increase of output to the increase of thesignal level.

Although the curved line a1 is described here, a function such as abroken line a1′ represented by a continuous line in FIG. 2 may be usedfor the curved line a1 instead. A formula representing the broken linea1′ is as below.

K=0.5×Y (Signal level of input video signal ≦50%)

K=1.5×Y−50 (Signal level of input video signal >50%)

Y: Signal level of input video signal (%)

K: Output

The curved line a2 shown in FIG. 3 is nearly a straight line having thesame gradient as the line b in a range of signal level between 0% and45%. The curved line a2 is a function plotting a line which is curvedupward in relation to the line b in the range of signal level between45% and 100%. An output is equal to a signal level in a range betweensignal levels 0% and 45%. In a range above signal level 45%, an outputis gradually raised to a bigger value than a signal level andapproximated to 100 earlier than the line b.

The curved line a3 in FIG. 4 represents a function plotting a line whichis, in relation to the line b, curved downward in a range between signallevels 0% and 50% and curved upward in a range more than signal level50%. When an inflexion point is set to signal level 30%, within a rangeless than signal level 30%, an output is gradually raised up to ½ ofsignal level suppressing the increase of the output in relation to theincrease of the signal level. In a range equal to or more than signallevel 30%, the output is raised at once in order that the output may be50 at signal level 50%. An inflexion point is set to signal level 70% ina range of signal level more than 50%. An output is raised up to 85 atonce in a range of signal level between 50% and 70%. In a range ofsignal level more than 70%, the output is gradually approximated to 100while suppressing the increase of the output in relation to the increaseof the signal level.

The image characteristic value determination section 20 takes an averageof an output inputted from the weighting section 19. An imagecharacteristic value of one frame is calculated here in such a mannerthat outputs of one frame are accumulated and the accumulated result isdivided by the number of total pixels on a display screen. As well as anoutput, an image characteristic value is expressed by a value between 1and 100.

Although average luminance levels (APL) of an image consisting of thedarkest half-screen part and the brightest half-screen part, and a gray(50%) full-screen image are the same, a difference between the imagecharacteristic values thereof makes it possible to distinguish theimages from each other because the image characteristic values foundafter weighting the luminance information of video signals are not thesame.

The backlight luminance determination section 21 in the backlightcontrolling section 13 determines the luminance of the backlight 12 inaccordance with an image characteristic value inputted from the imagecharacteristic value determination section 20 and outputs the luminanceas a backlight luminance signal. The backlight luminance determinationsection 21 is provided with formulas which represent the relationsbetween an image characteristic value and the luminance of the backlight12 as in FIGS. 5 through 7 or tables, which represents the relations.With the use of the formula or the table, the backlight luminancedetermination section 21 determines the luminance of the backlight ofthe backlight 12 in accordance with an image characteristic valueinputted from the image characteristic value determination section 20.

The horizontal axes in FIGS. 5 through 7 represent the imagecharacteristic values inputted from the image characteristic valuedetermination section 20, in other words, averages each of which is thequotient worked out by dividing, by the number of total pixels on adisplay screen, accumulation of outputs outputted from the weightingsection 19 per one frame. On the other hand, the vertical axes in FIGS.5 through 7 represent the luminance of the backlight 12 which isoutputted to the backlight controlling section 13 as a backlightluminance signal. The luminance of the backlight 12 which is expressedin percentage is 100% in the brightest state that the amount ofillumination light is the largest whereas the luminance is 0% in the OFFstate that the amount of illumination light is zero.

In the relation in FIG. 5, a luminance of a backlight is kept at 100%while an image characteristic value increases from 0 to the firstpredetermined value (i.e. image characteristic value 30 in FIG. 5).After the image characteristic value surpasses the first predeterminedvalue, the luminance is lowered from 100% to, at image characteristicvalue 90, around 40%. In the range more than image characteristic value90, the luminance is kept at 40% nearly constant.

With a formula or a table representing such a relation, the luminance ofthe backlight 12 is lowered at the image display of a bright image suchthat the most part of a display screen is displayed white. Thereby, itis possible to prevent giving a user a feeling of an image to be toobright, or to make it difficult to give him such a feeling. In addition,the reduction of power consumption is realized.

In the relation in FIG. 6, a luminance of the backlight is raised from50% to 100% while an image characteristic value increases from 0 to thesecond predetermined value (i.e. image characteristic value 17 in FIG.6). In the range of image characteristic value more than the secondpredetermined value, the luminance is kept at 100%.

With a formula or a table representing such a relation, lowering theluminance of the backlight 12 makes it possible to avoid powerconsumption for unnecessary emission of light at the image display of adark image such that the most part of a display screen is black andremedy the problem of a graying of black level due to the light leakageof the backlight 12 from a black display screen part.

The relation in FIG. 7 is such that the relations in FIGS. 5 and 6 arecombined. A luminance of the backlight is raised from 50% to 100% whilean image characteristic value increases from 0 to the secondpredetermined value (i.e. image characteristic value 17 in FIG. 7). Theluminance is kept at 100% until the image characteristic value surpassesthe first predetermined value (i.e. image characteristic value 30 inFIG. 7) in the range of image characteristic value more than the secondpredetermined value. After the image characteristic value surpasses thefirst predetermined value, the luminance is lowered from 100%.

With a formula or a table representing such a relation, because theluminance of the backlight 12 is lowered at the image display of a darkimage such that the most part of a display screen is displayed black andthe image display of a bright image such that the most part of a displayscreen is displayed white, it is possible to avoid power consumption forunnecessary emission of light and remedy the problem of a graying ofblack level in a black display screen part and the problem of a toobright image in a case where a white display screen part is large.

The first and the second predetermined values are preferably set to animage characteristic value between 10 and 30. The first and the secondpredetermined values may be the same (the first predetermined value ≧the second predetermined value).

In the relations in FIGS. 5 through 7, the luminance of the backlight 12is changed to draw a curve. However, the relations may be linear as inFIG. 8. In FIG. 8, the first predetermined value is set to 30; Thesecond predetermined value is set to 10.

The backlight luminance determined in such a way by the backlightluminance determination section 21 is outputted as a backlight luminancesignal. The backlight luminance signal is inputted into the PWMconverting section 18 via the temporal filter 17.

FIG. 9 illustrates an example in which the temporal filter 17 makes theluminance of the backlight 12 smoothly change. In this example, it takes6 frames in order to change the luminance of the backlight 12 from thecurrent 50% to the next target 100%. Specifically, an average of a valueof a current frame and a target value is taken and set as a value of thenext frame. By repeating this operation six times, a value reaches thetarget value in the sixth frame. Thus, by taking an average of aprevious and a target values, the change of the luminance becomesaccompanied by temporal delay.

In the example of FIG. 9, an average of a previous and a target valueswhich are equally weighted is taken. By changing the weighting, thetemporal delay can be controlled. That is, taking an average of aprevious and a target values with weighting makes it possible to controlthe number of frames required to achieve a target value.

As described above, in the image display device of the presentembodiment, the weighting section 19 weights the luminance informationof an input video signal; the image characteristic value determinationsection 20 takes an average of weighted luminance information by everyframe and determines an image characteristic value of one frame; and thebacklight controlling section 13 performs luminance correction of thebacklight 12 in accordance with the determined image characteristicvalue.

In this arrangement, not by a means in which histogram distribution isdetermined and the distribution pattern thereof is recognized, but by asimple means in which an average is taken, it is possible to distinguishdifferences between images which cannot be simply determined solely fromthe detected average luminance level and perform more effectiveluminance control according to characteristics of an image.

As for an area in which the weighting section 19 weights the luminanceinformation of an input video signal, it may be arranged such that, asin FIG. 10 for example, only the luminance information corresponding toa specific area in a full screen is weighted although the luminanceinformation of all the pixels of one frame of an input video signal inthe full screen of the LCD panel 11 is weighted above.

In the arrangement of FIG. 10, only the luminance informationcorresponding to the central area of the display screen except the 10%(dimensional ratio) top/bottom/left/right margins. In this case, thespecific area accounts for 64% of the full screen. In a case where aspecific area at the center of the screen is weighed, weighing an areaaccounting for 50%-70% as in FIG. 10 is effective in order to make aneffective contribution to image display.

In the case of, for example, dual window display as in FIG. 11( a), thearea of the main window (area ratio 50%-75%) in the two windows ispreferably specified as a specific area. In the case of Cinema Scopesize display, the display area (area ratio 74.5%-96.1%) is preferablyspecified as a specific area. Also, in the case of image display of a4:3 image as in FIG. 11( c) on a 16:9 display screen with the aspectratio of the image retained, the display area (area ratio 75%) ispreferably specified as a specific area. That is, it is preferablyarranged such that a target domain for determining an imagecharacteristic value is variable depending on a video source and adisplay mode.

In a case where a specific area in a full screen is a target domain forweighting, the image characteristic value determination section 20 inthe following stage determines an average by dividing the accumulationof outputs from the weighting section 19 by the number of pixels in aspecific area, needless to say.

In the image display device of the present embodiment, as described, thedisplay controlling section 14 does not use an image characteristicvalue determined by the image characteristic value determination section20 at outputting an input video signal as a panel driving signal to theLCD panel 11. However, in an arrangement such that utilizing a tablewhich defines a relation as shown in FIGS. 6 through 8, the backlightluminance determination section 21 lowers (less than 100%) the luminanceof the backlight 12 if an image characteristic value inputted by theimage characteristic value determination section 20 is equal to or lessthan the second predetermined value, the arrangement may be such thatthe luminance of the backlight 12 is lowered and a signal level iscorrected in order that the signal level of a video signal may beraised.

As represented by a dashed line in FIG. 1, such an arrangement isrealized by providing further a signal level correction section (a videosignal level correcting means) 22 to the display controlling section 14.A determined image characteristic value is inputted into the signallevel correction section 22 from the image characteristic valuedetermination section 20. The signal level correction section 22corrects a signal level in accordance with an inputted imagecharacteristic value in order that the signal level of a video signalmay rise in the case of the decrease of the luminance of the backlight12.

Because of this, decrease of a screen luminance is restrained by theraise of a signal level of an input video signal even if the luminanceof the backlight 12 decreases. As a result, an influence due to decreaseof the luminance of the backlight 12 hardly appears on a display screen.

In such an arrangement that the luminance of the backlight 12 is loweredand a signal level of the input video signal is raised by the displaycontrolling section 14 when an image characteristic value inputted fromthe image characteristic value determination section 20 is equal to orless than the second predetermined value, it is preferable that, whenthe image characteristic value is equal to or less than the secondpredetermined value, a peak luminance level or an average luminancelevel is set to an almost same level between both cases where there isno correction to both the backlight 12 and the input video signal andwhere there is no correction thereto. This makes it possible to preventmore effectively the influence due to the decrease of the luminance ofthe backlight 12 from appearing in a screen display.

Embodiment 2

An image display device of Embodiment 2 of the present invention isdescribed below with reference to FIGS. 12 and 13. For the sake ofsimplicity, members with same functions as those of members ofEmbodiment 1 are given the same symbols and descriptions for the membersare omitted.

FIG. 12 is a block diagram of a structure of the image display device ofEmbodiment 2 of the present invention. As illustrated in FIG. 12, theimage display device additionally includes a luminance sensor 23 whosemeasurement output is inputted into a backlight luminance determinationsection 21′ in a backlight controlling section 13′. The backlightcontrolling section 13′ is different from the backlight controllingsection 13 only in that the backlight controlling section 13′ includesthe backlight luminance determination section 21′ instead of thebacklight luminance determination section 21.

The luminance sensor 23 is for the measurement of luminance around theimage display device. A measurement output of the luminance sensor 23 isbetween luminance 0 and 100. A luminance of the very dark vicinity ofthe image display device is 0 whereas a luminance of the very brightvicinity of the image display device is 100.

The backlight luminance determination section 21′ takes into account themeasurement output which is inputted from the luminance sensor 23 inorder to determine the luminance of the backlight 12 in accordance withan image characteristic value inputted from the image characteristicvalue determination section 20. Only this is the difference between thebacklight luminance determination section 21′ and the backlightluminance determination section 21.

The backlight luminance determination section 21′, as in FIG. 13,includes a plurality of formulas or a plurality of tables, so that aformula or table most suitable for certain illuminance can be used forthe illuminance. The formulas or tables show the relationship betweenthe image characteristic values and the luminance of the backlight 12.The backlight luminance determination section 21′ selects a formula or atable according to luminance and thereby determines the luminance of thebacklight 12.

In FIG. 13, when luminance which is a measurement output of theluminance sensor 23 is 50, the backlight luminance determination section21′ uses a formula or a table whose relation between an imagecharacteristic value and the luminance of the backlight 12 is the sameas that in FIG. 8. On the other hand, when luminance is 100, theluminance of the backlight 12 is constantly kept at 100%. When luminanceis 0, on the other hand, the luminance of the backlight 12 is constantlykept at 50%. Between luminance 50 and 100, the minimum luminance of thebacklight 12 is raised according to luminance; between luminance 0 and50, the maximum luminance of the backlight 12 is lowered according toluminance.

In a case, for example, where the vicinity of the image display deviceis very bright, even a bright image which is largely displayed white isunclear to see due to the luminance of the vicinity with the decrease ofthe luminance of the backlight 12 and the decrease of screen luminancethereby. The arrangement makes it possible to prevent such a case of anunclear image due to the decrease of screen luminance because there isno or small decrease of the luminance of the backlight 12 according tothe luminance thereof when the vicinity of the image display device isbright.

On the other hand, in the case of the very dark vicinity of the imagedisplay device, a white display screen part tends to be perceived to betoo bright. In addition, a graying of black level in a black displayscreen part tends to be also noticeable. In this arrangement, a toobright image and a noticeable graying of black level can be avoided in acase where the vicinity of the image display device is dark because,according to the darkness, the luminance of the backlight 12 is loweredindependently of images or the peak luminance of the backlight 12 islowered. Besides, more efficient power saving is realized.

Embodiment 3

An image display device of Embodiment 3 of the present invention isdescribed below with reference to FIGS. 14 through 16, FIG. 17( a), andFIG. 17( b). For the sake of simplicity, members with same functions asthose of members of Embodiments 1 and 2 are given the same symbols anddescriptions for the members are omitted.

FIG. 14 is a block diagram of a structure of the image display device ofEmbodiment 3 of the present invention. As in FIG. 14, the image displaydevice additionally includes a histogram determination section (a meansfor determining histograms) 26. A histogram determination outputdetermined by the histogram determination section 26 is inputted intothe weighting section 19.

The histogram determination section 26 is for dividing luminanceinformation of one frame of an input video signal (a pixel value) into aplurality of luminance information classifications and determininghistogram distribution corresponding to each luminance informationclassification. The histogram determination section 26 classifies asignal level indicating the luminance information of all the pixels inone frame of an input video signal into classifications of level anddetermines histogram distribution therewith.

FIG. 15 shows an example of determined histogram distribution of oneframe of an image (276 pixels). In FIG. 15, the luminance information ofan input video signal (This is equivalent to the signal level) between 0and 100% is, by 5%, classified into 20 classifications of level intotal.

The weighting section 19, as stated, weights determined histogramdistribution with the function. The image characteristic valuedetermination section 20 accumulates outputs of the weighting section 19and divides the accumulated outputs by the number of all the pixels (atotal of frequencies) in order to calculate an average, i.e., an imagecharacteristic value.

FIG. 16 shows results which are the averages calculated on the basis ofweighted histogram distribution of FIG. 15. Factors in FIG. 16 are thefactors of the functions used by the weighting section 19. In theconventional cases without weighting, a median of each classification ofluminance value is the factor of the classification. In theclassifications in which the luminance values are equal to or less than50, the factor of the present invention is one half of a conventionalfactor whereas, in the classifications in which the luminance values aremore than 50, the factor is a value which is 1.5 times of theconventional factor minus 50.

That is, the conventional factor Y and the factor of the presentinvention K are represented as below.

K=0.5×Y (Luminance value ≦50%)

K=1.5×Y−50 (Luminance value >50%)

As shown in FIG. 16, an average (an average luminance value) calculatedon the basis of histogram distribution without weighting is 30.0. Incontrast, an image characteristic value which is an average of weightedhistogram distribution is 15.2. Thus, the weighting on histogramdistribution makes it possible to make a difference between theaverages.

A case of another image consisting of the darkest half-screen part andthe brightest half-screen part is described below. As shown in FIG. 17(a), with regard to frequency distribution of such an image, each halfthe number of pixels: 138 is included in the classification of level 0to 5 and the classification of level 95 to 100 respectively. As a resultof calculation of an average based on this, without weighting, theaverage luminance value is 50. The image characteristic value of theimage display device which performs weighting is 48.8.

A case of a gray (50%) full-screen image is described below. As shown inFIG. 17( b), with regard to frequency distribution of such an image, thenumber of all the pixels 276 is included in the classification of level45 to 50. As a result of calculation of an average based on this,without weighting, the average luminance value is 47.5. The imagecharacteristic value of the image display device which performsweighting is 23.8.

Thus, the weighting makes a big difference between the average luminancevalue and the image characteristic value and thereby makes it possibleto easily distinguish even images which are difficult to distinguishfrom each other based on average luminance values thereof due to almostthe same average luminance values, such as the image consisting of thedarkest half-screen part and the brightest half-screen part, and thegray (50%) full-screen image.

Therefore, appropriate luminance control according to characteristics ofan image can be performed with characteristics of the image extractedthrough a simple processing in which weighting is performed on histogramdistribution and taking an average of the result of the weightingwithout performing pattern recognition on determined histogramdistribution through image processing as in a conventional manner.

It may be arranged such that a weighted value used by the weightingsection 19 is switched to a more appropriate value according to thestate of determined histogram distribution. Switching the weighted valueenables more appropriate control according to characteristics of animage.

For example, widths of histogram distributions are different between thetwo cases above. Contrast of an image is very high when the width ofhistogram distribution is wide as in the case of the image consisting ofthe darkest half-screen part and the brightest half-screen part. On theother hand, contrast of an image is very low when histogram distributionconcentrates on a narrow range as in the case of the gray (50%)full-screen image.

Therefore, it becomes possible to adjust the luminance of the backlight12 more appropriate to characteristics of an image by the determinationof contrast of the image according to the width of histogramdistribution and the switching of the weighting value in order toperform luminance control according to the contrast of the image.

As shown in FIGS. 6 and 7, the backlight luminance determination section21 performs luminance correction in order that the luminance of thebacklight 12 may decrease when the image characteristic value is equalto or less than the second predetermined value only in a case where thenumber of pixels in the luminance information classification whoseluminance information is equal to or more than a predetermined luminanceinformation of determined histogram distribution is equal to or lessthan a predetermined value.

For example, in the case of an image such that the most part of adisplay screen displays black and a part thereof displays white,characteristics of the image can be determined with histogramdistribution. Therefore, in the case of an image such that the most partof the display screen is black and a part thereof is white, by notlowering the luminance of the backlight 12, it is possible toeffectively avoid a problem in that the screen luminance of the whitedisplay screen part decreases due to an effect of the decrease of theluminance of the backlight 12.

Embodiment 4

An image display device of Embodiment 4 of the present invention isdescribed with reference to FIG. 18. For the sake of simplicity, memberswith same functions as those of members of Embodiments 1 through 3 aregiven the same symbols and descriptions for the members are omitted.

FIG. 18 is a block diagram of a structure of the image display device ofEmbodiment 4 of the present invention. As illustrated in FIG. 18, theimage display device of the present embodiment is different from that ofEmbodiment 3 only in that the image display device of the presentembodiment additionally includes the luminance sensor 23. An measurementoutput of the luminance sensor 23 is, as described for the image displaydevice of Embodiment 2, inputted into the backlight luminancedetermination section 21′ in the backlight controlling section 13′.

This arrangement makes it possible to perform more appropriate luminancecorrection because luminance correction of a light source taking intoaccount the luminance in the vicinity of the image display device can beperformed as well as the image display device of Embodiment 2.

The image display device of the present invention may be arranged suchthat the weighting means performs weighting, for example, by using afunction of the luminance information of the inputted video signal.

The image display device of the present invention may be furtherarranged such that the weighting means uses a function of the luminanceinformation of the inputted video signal and selects the functionaccording to the histogram distribution of the luminance informationclassifications, the histogram distribution being determined by thehistogram determining means. Shifting the weighting value enables moreappropriate control according to characteristics of an image.

The image display device of the present invention may be furtherarranged such that the weighting means performs weighting in such amanner that luminance information of higher luminance side is weightedrelatively more than luminance information of lower luminance side. Suchweighting makes it possible to efficiently differentiate images whichare difficult to differentiate based on the average luminance levels(APL) thereof.

The image display device of the present invention may be furtherarranged such that the light source controlling means performs suchluminance correction that the luminance of the light source is decreasedwhen the image characteristic value determined by the imagecharacteristic value determining means is equal to or more than thefirst predetermined value.

The luminance of the light source is lowered when displaying a brightimage that the most part of a display screen is white. This makes ithard or impossible to give a user such a feeling that an image to be toobright. In addition, the power consumption can be reduced.

The image display device of the present invention may be furtherarranged such that the light source controlling means performs suchluminance correction that the luminance of the light source is decreasedwhen the image characteristic value determined by the imagecharacteristic value determining means is equal to or less than thesecond predetermined value.

According to this arrangement, at the image display of a dark image suchthat the most part of a display screen is black, lowering the luminanceof the light source makes it possible to avoid power consumption due tounnecessary emission of light and remedy the problem of a graying ofblack level due to the leakage of the light of the light source from ablack display screen part.

The image display device of the present invention may be furtherarranged such that the light source controlling means performs suchluminance correction that the luminance of the light source is decreasedwhen the image characteristic value determined by the imagecharacteristic value determining means is equal to or less than thesecond predetermined value in a case where the number of pixels in theluminance information classifications whose luminance information isequal to or more than predetermined luminance information in thehistogram distribution corresponding to each luminance informationclassification determined by the histogram determining means is equal toor less than a predetermined value.

For example, in the case of such an image that the most part of adisplay screen is black and a part thereof is a white display screenpart, a viewer would feel the image to be unclear due to the decrease ofscreen luminance in the white display screen part affected by thedecrease of the luminance of the light source. With this arrangement,such a problem is avoided because the light source controlling meansdoes not perform the decrease of the luminance of the light source withextraction of characteristic of such an image.

The image display device of the present invention may further include: avideo signal level correction means for correcting a signal level of avideo signal in such a manner that the signal level is raised, the videosignal being inputted thereto and being to be supplied to the lightmodulation device, the light source controlling means performing theluminance correction of the light source in such a manner that theluminance of the light source is decreased, and the video signal levelcorrection means performing the correction of the signal level in such amanner that the signal level of the video signal is raised.

For example, in the case of an image such that the most part of adisplay screen is black and a part thereof is a white display screenpart, a viewer would feel the image to be unclear due to the decrease ofscreen luminance in the white display screen part affected by thedecrease of the luminance of the light source. With this arrangement,such a problem is avoided because a signal level of an input videosignal is raised even if the luminance of the light source is lowered.In this case, specifically, it is possible to more effectively preventan influence of the decrease of the luminance of the light source to bedisplayed on a display screen by performing the luminance correction andthe correction to the signal level in order that a peak luminance levelor an average luminance level of the light modulation device may beconstant before and after the luminance correction of the light sourceand the correction to the signal level of the video signal.

The image display device of the present invention may further includes:a luminance sensor for detecting luminance in the vicinity of the imagedisplay device, the light source controlling means changing degree ofthe correction in accordance with an output of the luminance sensor.

This makes it possible to perform more appropriate luminance correctionbecause luminance correction of the light source taking into account theluminance in the vicinity of the image display device can be performed.

Specifically, in an arrangement that a degree of the luminancecorrection is changed in accordance with an output of the luminancesensor, the light source controlling means preferably keeps theluminance of the light source at a constant value in spite of theinputted video signal when the luminance sensor detects that theluminance in the vicinity of the image display device is higher than apredetermined value.

For example, in a case where the vicinity of the image display device isvery bright, even a bright image which is largely white is unclear tosee due to the luminance of the vicinity with the decrease of screenluminance. However, this arrangement makes it possible to prevent theimage to be unclear to see because the reduction of the luminance of thelight source is not performed when the vicinity of the image displaydevice is bright.

The image display device of the present invention may be arranged suchthat the weighting means weights luminance information that correspondsto a specific area in a full screen of the inputted video signal.

This makes it possible to perform more appropriate luminance correctionbecause the luminance correction of the light source can be performedbased on luminance of an area which effectively contributes to imagedisplay.

The image display device of the present invention may be arranged suchthat the histogram determining means determines the histogramdistribution according to luminance information that corresponds to aspecific area in a full screen of the input video signal.

This makes it possible to perform more appropriate luminance correctionbecause the luminance correction of the light source can be performedbased on luminance of an area which effectively contributes to imagedisplay.

INDUSTRIAL APPLICABILITY

This invention is applicable to, for example, a liquid crystal displaydevice which performs moving image display.

1. An image display device comprising a light source and a lightmodulation device which attenuates light from the light source inaccordance with a video signal so as to change the luminance of adisplay image, the image display device, comprising: weighting means forweighting luminance information of the video signal inputted thereto;image characteristic value determining means for determining an imagecharacteristic value of one frame by taking by one frame an average ofthe luminance information weighted by the weighting means; and lightsource controlling means for performing luminance correction of thelight source in accordance with the image characteristic valuedetermined by the image characteristic value determining means.
 2. Animage display device which displays an image of an inputted video signalon a light modulation device having a light source, comprising:histogram determining means for dividing luminance information of oneframe of the inputted video signal into a plurality of luminanceinformation classifications and determining histogram distribution ofthe luminance information classifications; weighting means for weightingthe histogram distribution of the luminance information classifications,the histogram distribution being determined by the histogram determiningmeans; image characteristic value determining means for determining animage characteristic value by taking an average of a result of thehistogram distribution of the luminance information classifications, thehistogram distribution being weighted by the weighting means; and lightsource controlling means for performing luminance correction of thelight source in accordance with the image characteristic valuedetermined by the image characteristic value determining means.
 3. Theimage display device as set forth in claim 1 wherein the weighting meansperforms weighting by using a function of the luminance information ofthe inputted video signal.
 4. The image display device of claim 2wherein: the weighting means uses a function of the luminanceinformation of the inputted video signal and selects the functionaccording to the histogram distribution of the luminance informationclassifications, the histogram distribution being determined by thehistogram determining means.
 5. The image display device as set forth inclaim 1 wherein the weighting means performs weighting in such a mannerthat luminance information of higher luminance side is weightedrelatively more than luminance information of lower luminance side. 6.The image display device as set forth in claim 1 wherein the lightsource controlling means performs such luminance correction that theluminance of the light source is decreased when the image characteristicvalue determined by the image characteristic value determining means isequal to or more than the first predetermined value.
 7. The imagedisplay device as set forth in claim 1 wherein the light sourcecontrolling means performs such luminance correction that the luminanceof the light source is decreased when the image characteristic valuedetermined by the image characteristic value determining means is equalto or less than the second predetermined value.
 8. The image displaydevice as set forth in claim 2 wherein the light source controllingmeans performs such luminance correction that the luminance of the lightsource is decreased when the image characteristic value determined bythe image characteristic value determining means is equal to or lessthan the second predetermined value in a case where the number of pixelsin the luminance information classifications whose luminance informationis equal to or more than predetermined luminance information in thehistogram distribution corresponding to each luminance informationclassification determined by the histogram determining means is equal toor less than a predetermined value.
 9. The image display device as setforth in claim 7, comprising: a video signal level correction means forcorrecting a signal level of a video signal in such a manner that thesignal level is raised, the video signal being inputted thereto andbeing to be supplied to the light modulation device, the light sourcecontrolling means performing the luminance correction of the lightsource in such a manner that the luminance of the light source isdecreased, and the video signal level correction means performing thecorrection of the signal level in such a manner that the signal level ofthe video signal is raised.
 10. The image display device as set forth inclaim 9 wherein the light source controlling means and the video signallevel correction means perform the luminance correction and thecorrection of the signal level in order that a peak luminance level ofthe light modulation device may be constant before and after theluminance correction of the light source and the correction to thesignal level of the video signal when the image characteristic valuedetermined by the image characteristic value determining means is equalto or less than the second predetermined value.
 11. The image displaydevice as set forth in claim 9 wherein the light source controllingmeans and the video signal level correction means perform the luminancecorrection and the correction of the signal level in order that anaverage luminance level of the light modulation device may be constantbefore and after the luminance correction of the light source and thecorrection to the signal level of the video signal when the imagecharacteristic value determined by the image characteristic valuedetermining means is equal to or less than the second predeterminedvalue.
 12. The image display device as set forth in claim 11,comprising: a luminance sensor for detecting luminance in the vicinityof the image display device, the light source controlling means changingdegree of the correction in accordance with an output of the luminancesensor.
 13. The image display device as set forth in claim 12 whereinthe light source controlling means keeps the luminance of the lightsource at a constant value in spite of the inputted video signal whenthe luminance sensor detects that the luminance in the vicinity of theimage display device is higher than a predetermined value.
 14. The imagedisplay device as set forth in claim 1 wherein the weighting meansweights luminance information that corresponds to a specific area in afull screen of the inputted video signal.
 15. The image display deviceas set forth in claim 2 wherein the histogram determining meansdetermines the histogram distribution according to luminance informationthat corresponds to a specific area in a full screen of the input videosignal.
 16. The image display device as set forth in claim 2 wherein theweighting means performs weighting by using a function of the luminanceinformation of the inputted video signal.
 17. The image display deviceas set forth in claim 2 wherein the weighting means performs weightingin such a manner that luminance information of higher luminance side isweighted relatively more than luminance information of lower luminanceside.
 18. The image display device as set forth in claim 8, comprising:a video signal level correction means for correcting a signal level of avideo signal in such a manner that the signal level is raised, the videosignal being inputted thereto and being to be supplied to the lightmodulation device, the light source controlling means performing theluminance correction of the light source in such a manner that theluminance of the light source is decreased, and the video signal levelcorrection means performing the correction of the signal level in such amanner that the signal level of the video signal is raised.
 19. Theimage display device as set forth in claim 2, comprising: a luminancesensor for detecting luminance in the vicinity of the image displaydevice, the light source controlling means changing degree of thecorrection in accordance with an output of the luminance sensor.